1. Field of the Invention
The present invention relates to a thermal infrared detector having a thermal isolation structure for minimum temperature resolution.
2. Description of the Related Art
Thermal infrared detectors are available in different types including a bolometer type, a pyroelectric type, and a thermocouple type. These thermal infrared detectors have a thermal isolation structure, i.e., a so-called diaphragm structure, for increased detection sensitivity. In the thermal isolation structure, a diaphragm has an outer edge supported in spaced relation to a substrate by a plurality of beams.
Several thermal isolation structures of conventional thermal infrared detectors will be described below with reference to accompanying drawings.
FIG. 1 of the accompanying drawings schematically shows in plan a pixel of a 320xc3x97240 pixel thermistor bolometer-type infrared detector developed by Santa Barbara Research Center (see W. Radford et al., SPIE, Vol. 2746, 1996, page 82). The pixels of the illustrated infrared detector are spaced at a pitch of 48 xcexcm. Each of the pixels comprises a substrate 101 on which a readout circuit is formed, and a diaphragm (photo-sensitive area) 102 supported on and spaced from the substrate 101 by two beams 103. The entire chip of the infrared detector, which includes a matrix of those pixels, is encased in an evacuated package. The diaphragm 102 comprises a thin film of thermistor bolometer material and a protective film surrounding the thin film of thermistor bolometer material. Each of the beams 103 comprises an interconnection which electrically connects the readout circuit to the thin film of thermistor bolometer material via a contact 105, and a protective film surrounding the interconnection.
When an infrared radiation is applied to the diaphragm 102, the applied infrared radiation is absorbed by the diaphragm 102, producing heat which increases the temperature of the diaphragm 102. The rise in the temperature of the diaphragm 102 changes the resistance of the thin film of thermistor bolometer material, and the changed resistance is converted by the readout circuit in the chip into an electric signal, which is converted into an image representative of the detected infrared radiation.
In order to increase the sensitivity of the infrared detector, it is important that the heat absorbed by the diaphragm 102 and transferred to the substrate 101 be minimized, i.e., the thermal conductance of the diaphragm 102 be reduced. In many thermal infrared detectors, the chip is encased in the evacuated package and the beams 103 be thinned to reduce the thermal conductance of the diaphragm 102.
FIG. 2 of the accompanying drawings schematically shows in plan a pixel of a 320xc3x97240 pixel thermistor bolometer-type infrared detector developed by Loral Infrared and Imaging Systems (see C. Marshall et al., SPIE, Vol. 2746, 1996, page 23). The pixels of the illustrated infrared detector are spaced at a pitch of 46.25 xcexcm. Each of the pixels comprises a substrate 111, and a diaphragm 112 supported on and spaced from the substrate 111 by two beams 113, and contacts 115. The infrared detector shown in FIG. 2 detects an infrared radiation according to the same principle as the infrared detector shown in FIG. 1. The infrared detector shown in FIG. 2 differs from the infrared detector shown in FIG. 1 in that the beams 113 are bent around an outer peripheral edge of the diaphragm 112 and the beams 113 are longer than the beams 103 shown in FIG. 1. The diaphragm 112 thus constructed has a small thermal conductance for increased detection sensitivity.
FIGS. 3a through 3e of the accompanying drawings schematically show in plan a pixel of a thermistor bolometer-type infrared detector developed by National Optics Institute (see H. Jerominek et al., SPIE, Vol. 2746, 1996, page 60). The infrared detector shown in FIGS. 3a through 3e has a diaphragm 122 supported on a substrate 121 by beams 123, and detects an infrared radiation according to the same principle as the infrared detector shown in FIG. 1. FIGS. 3a through 3e show various different thermal isolation structures. Specifically, the thermal isolation structures shown in FIGS. 3a, 3b differ from each other with respect to the number of bends of beams 123. The thermal isolation structure shown in FIG. 3b has a better thermal isolation capability than the thermal isolation structure shown in FIG. 3a, but has a smaller filling factor of the diaphragm 122, i.e., a smaller occupation ratio of the diaphragm 122 with respect to the pixel. The thermal isolation structure shown in FIG. 3c is designed to increase the thermal isolation capability by changing the manner in which the beams 123 are bent. Each of the thermal isolation structures shown in FIGS. 3d, 3e has four beams 123 to support the diaphragm 122.
FIG. 4a of the accompanying drawings schematically show in plan a pixel of a 16xc3x9716 pyroelectric infrared detector developed by Toyota Central R and D Labs., Inc., and FIG. 4b is a cross-sectional view taken along line 4bxe2x80x944b of FIG. 4b (see Fujitsuka et al., Journal of Japan Society of Infrared Science and Technology).
In FIGS. 4a and 4b, the pixel has sides each 75 xcexcm long, and includes a cavity 205 defined in an n-type silicon substrate 200 by the bulk micro-machining technology. A diaphragm 201 is supported over the cavity 205 by four beams 202 each having a width of 4 xcexcm and a length of 59 xcexcm. The diaphragm 201 comprises a silicon oxide film 203 and an electrode 204 of Ti/TiN. One of the four beams 202 has an interconnection 204xe2x80x2 of Ti/TiN that is electrically connected to the electrode 204. Although not shown, a thin film of PVDF (Polyvinylidene Fluoride) as a pyroelectric material is deposited on the electrode 204 of the diaphragm 201, and a metal film serving as an infrared absorption film and also as an upper electrode is deposited on the thin film of PVDF.
An infrared radiation applied to the diaphragm 201 is absorbed by the diaphragm 201, producing heat that changes the polarized state of the pyroelectric material and generates a surface charge on the diaphragm 201. The surface charge is converted by a readout circuit (not shown) on the n-type silicon substrate 200 into an electric signal, which is converted into an image representative of the detected infrared radiation.
While various thermal isolation structures have heretofore been attempted, they have not been optimized to minimize the temperature resolution. Specifically, the temperature resolution can generally be reduced by increasing the length of the beams to provide a better thermal isolation between the diaphragm and the substrate. However, if the pixels are large, then excessively increasing the thermal isolation results in a worse temperature resolution because it increases a thermal time constant that prevents the diaphragm from keeping up with a change in the temperature of the subject being detected.
It is therefore an object of the present invention to provide a thermal infrared detector having beams whose lengths are optimum for minimizing the temperature resolution depending on the size of pixels.
To achieve the above object, a thermal infrared detector according to the present invention has a substrate having a readout circuit and a plurality of pixels patterned on the substrate at a pitch p in the range of 15xe2x89xa6pxe2x89xa650 (xcexcm). Each of the pixels has a diaphragm including a thin film of bolometer and spaced from the substrate, two beams by which the diaphragm is supported on the substrate, and interconnections formed respectively on the beams and connecting the readout circuit and the thin film of bolometer to each other. The length of each of the beams is determined by a beam length index which is produced by dividing the length of each of the beams by one-quarter of the peripheral length of the pixel. The beam length index is given by an approximate expression using the pixel pitch as a parameter, determined depending on the type of a stepper used to pattern the pixels and the thermal conductivity of the material of the interconnections based on an equation representing temperature resolution (the equation (1) described later on). In view of the patterning accuracy of the stepper, the beam length index is given in the range of xc2x115% of the above approximate expression.
By thus determining the length of the beams according to the beam length index that is determined using the pixel pitch as a parameter, the thermal infrared detector has a beam length optimum, given depending on the pixel pitch, for minimizing the temperature resolution.
The thermal conductivities k of interconnection materials of the beams are classified into a first group where 0.065xe2x89xa6kxe2x89xa60.09 (W/(cmxc2x7K)), a second group where 0.10xe2x89xa6kxe2x89xa60.15 (W/(cmxc2x7K)), and a third group where 0.16xe2x89xa6kxe2x89xa60.22 (W/(cmxc2x7K)). The interconnection materials of the first group include alloys mainly composed of Ti and containing at least Al, the interconnection materials of the second group include Nixe2x80x94Cr alloys, and the interconnection materials of the third group include Ti or alloys mainly composed of Ti and containing no Al.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.